Nature's nacreous (pearlescent) pigment, plate-like guanine microcrystals from fish, has only limited uses because of high cost, source variations and unsuitability for some applications. As a result, a number of synthetic nacreous pigments have been developed. One of the most important of these, titanium dioxide coated mica, consists of mica platelets having adherent crystalline TiO.sub.2 coatings thereon. The composite pigment has good reflectivity characteristics, a high degree of stability with respect to heat and chemical agents, and it is non-toxic making it suitable for cosmetic applications. Such pigments in which the TiO.sub.2 is in the anatase form are described, for example, in Klenke, et al, U.S. Pat. No. 3,087,827; Linton U.S. Pat. No. 3,087,828; Rieger U.S. Pat. No. 3,418,146; and Quinn U.S. Pat. No. 3,437,515. The pigments in which the titanium dioxide coating is in the rutile crystalline form is described in DeLuca, et al, U.S. Pat. No. 4,038,099.
Metal oxides other than titanium dioxide have been used to prepare similar types of nacreous pigments by coating on mica platelets as a substrate. Examples include ferric oxide (Linton U.S. Pat. No. 3,087,829), zirconium dioxide (Linton U.S. Pat. No. 3,087,828), tin dioxide (Esselborn U.S. Pat. No. 4,040,859) and the like.
The metal oxide coating on the mica has a high refractive index and provides the optical effects, including high luster or reflectivity, coverage, interference reflection color (if the metal oxide coating is sufficiently thick), and absorption color (if the metal oxide contains color material). The mica, on the other hand, has a low refractive index and is essentially a carrier. It makes almost no contribution to the optical effects of the pigment. The weight of the mica in the pigment usually lies between 40 and 90% and most usually in the range of 60 to 80%.
Although procedures have been developed to classify and purify the mica, certain variations from the natural source persist through to the final product. Most sources of mica are unsuitable for use in nacreous pigments since they are quite dark or cannot be adequately ground to yield the desired dimensions for the pigment platelet substrates. Additionally, mica darkens considerably during the calcination process used to preparing the metal oxide coated mica product resulting in undesirable color effects.
In many applications, the incorporation of the pigment into the system to be colored thereby is limited by the weight of the pigment. Frequently, a maximum loading (concentration) is reached before an optimum in optical properties of the system is obtained.
Unsupported TiO.sub.2 pigments have also been disclosed in the technical literature. See, e.g., Haslam U.S. Pat. No. 2,941,895; Jenkins U.S. Pat. No. 3,018,186; Miller U.S. Pat. No. 3,071,482; Morita U.S. Pat. No. 3,395,203; Mochel U.S. Pat. No. 3,340,006; and Japan No. 35-15579 (275573). Generally, the methods of preparation of such pigments involve the hydrolysis of TiCl.sub.4 or a titanium ester on a flat surface during extrusion through a narrow orifice to form the desired flat shape of TiO.sub.2. The latter is produced in relatively small yield as an amorphous material containing hydrated forms of TiO.sub.2 as well as some contamination from the starting reactants. Such products are not very stable and react with the components of certain systems in which they are incorporated, such as nitrocellulose lacquers. Their light stability is also less than desirable, making them unsuitable for various applications in both cosmetics and plastics. Further, it is difficult to accurately regulate their thickness as is necessary in the preparation of nacreous pigments exhibiting predetermined interference colors. In addition, calcining unsupported TiO.sub.2 pigment platelets prepared as previously described results in a high degree of fracturing of the platelets, primarily due to the absence of a support or substrate therefor. Control over platelet thickness during such a calcination is difficult, if not impossible, to achieve.
Unsupported crystalline titanium dioxide platelet nacreous pigments are described by Waitkins, et al in U.S. Pat. No. 3,861,946. The unsupported nacreous pigment is achieved by coating titanium dioxide on gypsum platelets followed by dissolving away the calcium sulfate substrate. When the titanium dioxide is in the anatase form, the pigment contains titanium dioxide, silicon dioxide and calcium sulfate. When the titanium dioxide is in the rutile form, the pigment contains titanium dioxide, stannic oxide, aluminum oxide and calcium sulfate. The unsupported platelets of titanium dioxide formed as described in this patent do not have the high degree of luster displayed by titanium dioxide coated mica because the calcium sulfate platelet is not nearly as smooth as are the mica platelets.
It is among the objects of the present invention to provide an improved unsupported metal oxide nacreous pigment which can be used in a wide variety of cosmetic and plastic applications and which possesses improved stability, luster, mechanical integrity and other properties, as compared with previously proposed metal oxide nacreous pigments. A further object of the invention is to provide a method for the preparation of such pigments in high yield and with precise control over particle sizes and thicknesses which determine nacreous pigment characteristics. These and other objects and advantages of the invention will become apparent from consideration of the following detailed description of the preferred embodiments thereof.